Exotic
and Invasive Species

Invasive plant and animal speciesalso referred to as exotics, non-natives,
introduced, or nonindigenous speciesare organisms which have expanded
beyond their native range or have been introduced from other parts of
the world. Some species were introduced into the wild intentionally, while
others have been introduced unintentionally and expanded on their own.

Of particular concern on the Colorado Plateau are introduced weedy plants
which are invading rangelands, forests, and riparian ecosystems at an
alarming rate. Human activities such as grazing
of livestock or logging, with
its associated road networks, often
disturb biotic communities enough
to allow establishment and in some cases domination of invasive species.
Control of infestations has been difficult, and the ecological consequences
have been serious. Negative impacts include reduction in biodiversity,
forage, habitat and aesthetic quality, and even soil productivity. The
rapid expansion of exotic weed populations has been a deterrent to restoring
native plant communities and re-establishing historic ecological conditions.

One of the most important, and damaging, introduced species on the Colorado
Plateau is saltcedar, or tamarisk. Eight species of this small riparian
tree were purposely introduced in the western United States as ornamentals,
for windbreaks, or to help control streambank erosion. Some of these species,
principally Tamarix ramosissima, but also T. chinensis
and T. parviflora, have established themselves in nearly every
lower-elevation streambed from northern Mexico to southern Canada and
now cover approximately 1.5 million acres.

Native riparian cottonwood/willow
communities, which support some of the highest numbers of breeding
bird species found in any vegetative community type in the United States,
have declined dramatically as tamarisk has invaded. The plant, without
any native checks, has replaced thousands of acres of riparian gallery
forest, resulting in a significant decrease in biodiversity and ecosystem
health along most of the Colorado Plateau's waterways. Click
here for a more comprehensive essay on tamarisk by ecologist Dr. Larry
Stevens.

Cheatgrass (Bromus tectorum) is a winter annual grass which
originated in Europe and Asia (Eurasia) and came to our area in contaminated
seed in the 1890s. By 1920, cheatgrass, or downy brome as it also is known,
had invaded native semi-arid grasslands and open pinyon-juniper
woodlands of the Colorado Plateau. Despite its early growth and rich
color, cheatgrass is unpalatable to sheep and other livestock, which tend
to overgraze native plants when it begins to prevail.

Most native bunchgrasses of the Colorado Plateau are perennial, whereas
annual plants like cheatgrass grow from a seed, then flower, set seed,
and die every year. Cheatgrass usually germinates in fall and grows during
winter, opposite the cycle followed by common native perennial grasses.
By the time the rain stops in spring, cheatgrass already is maturing its
seeds. Unlike native bunchgrasses, cheatgrass then dies by the end of
July, avoiding the hottest and driest part of summer.

Dead cheatgrass burns easily, causing early and abundant wildfires which
tend to damage or kill native grasses. During a fire, early-maturing cheatgrass
seeds can take advantage of many nutrients the fire releases to grow large
and produce abundant seed (over a thousand per plant in some cases).

Because cheatgrass quickly develops a large root system in the spring,
by the time native grass seedlings start to grow in April or May, cheatgrass
has stolen most water out of the top foot of soil. Although mature native
grasses can get water from lower soil regions, seedlings cannot get their
roots deep enough into soil to access water before drought sets in, and
thus, die of thirst. Without this ability to reproduce, native grasses
inevitably decline, and so over time, cheatgrass becomes more and more
common until eventually it dominates. Cheatgrass often opens the way for
secondary invaders such as knapweed and thistle

Other harmful introduced species found in plant communities on the Colorado
Plateau include Dalmation toadflax (Linaria dalmatica), leafy
spurge (Euphorbia esula), and camelthorn (Alhagi pseudalhagi)),
which is primarily spread by livestock grazing and wind.

Southwest Exotic Plant Information
Clearinghouse (SWEPIC) is a cooperative effort among the U.S. Geological
Survey, the National Park Service and Northern Arizona University to organize
comprehensive information on exotic plant species in the southwest on
one web location. SWEPIC is to help all people and organizations committed
to protecting the ecological and economic values of southwest resources
from degradation from harmful non-native weeds. The goal of SWEPIC is
to provide reliable and organized information on the distribution and
ecology of these weeds in the southwest, with an emphasis on forests,
rangelands, and other natural areas.

DeLoach, C. J. 1990. Prospects for biological control of salt cedar (Tamarix
spp.) in riparian habitats of the southwestern United States. Pp. 307-314
In: Delfosse, E. S., editor. Proceedings of the 7th International
Symposium on biological control of weeds.

Douglas, E. 1954. Phreatophytes: Water hogs of the west. Land Improvement
1: 8-12.

Horton, J. S. 1977. The development and perpetuation of the permanent
tamarisk type in the phreatophyte zone of the southwest. Pp. 124-127
In: Importance, preservation, and management of riparian habitat: A symposium.
General Technical Report RM-43. U.S. Forest Service, Washington, D.C.

Kovalev, O. V., Zaitzev, V. F. and Hoffmann, J. H. 1996. A new theoretical
approach to the selection of promising agents for biological weed control.
Pp. 283-285 In: Moran, V. C., editor. Proceedings of the 9th
international symposium on biological control of weeds, Stellenbosch,
South Africa, 19-26 January 1996. University of Cape Town, Rondebosch,
South Africa.

Molles, M. 1980. The impacts of habitat alterations and introduced species
on the native fishes of the upper Colorado River basin. Pp. 163-181
In: Spofford, W. O., Jr., Parker, A. L. and Kneese, A. V., editors.
Energy development in the southwest. Research Paper 18. Resources
for the Future, Washington, D.C.